1. Field of the Invention
The present invention relates to mobile radio communications technologies.
2. Description of the Related Art
In recent years, studies have been made of a multi-hop relay communications system in order to cover an area where the quality of communications with a base station (BS) is poor. This is partly due to an increase in the need for providing cheaper high-capacity, high-speed data communications to end users.
Base stations are connected directly to an access network with lines through, for example, Ethernet (registered trademark). These lines include optical fibers. For example, a network is constructed as illustrated in FIG. 1. Referring to FIG. 1, an access network 10 is included in an IP core network. The IP core network includes IP routers and Ethernet lines. The access network 10 and radio base stations (BS#1, BS#2, and BS#3) are connected with IP routers, Ethernet lines and radio circuits. Mobile stations (MS) communicate with radio base stations through radio communications interfaces. The mobile stations include mobile terminals.
In the case of performing multi-hop relay communications, there is wireless access between radio base stations and mobile stations, and there is also wireless access between radio base stations and radio relay stations (RS) (RS#11, RS#12, RS#21, RS#22, and RS#31), which perform radio relaying and transferring. (For example, see Japanese Laid-Open Patent Application No. 2001-237764.) The radio relay stations are capable of further accessing other different radio relay stations. By thus performing communications between radio base stations and mobile stations through multiple radio relay stations, it is possible to perform radio relaying of cascading connection. There is constant wireless access between radio relay stations and radio base stations. The radio relay stations are statically installed in corresponding service areas.
For example, there is wireless access between each radio relay station and multiple mobile stations in a radio communications area covered by the radio relay station. Further, there is also wireless access between the radio relay station and a radio base station. The communications data transmitted by the mobile stations having wireless access to the radio relay station are received by the radio base station via the radio relay station. The radio base station transmits the received communications data to the core network.
Multi-hop relay communications dispense with a fiber connection to a radio base station, which is made in the case of installing an optical extension station extended with a line from a large-capacity radio base station. Therefore, it is possible to cover an area of a poor radio propagation environment, such as an urban area, in multi-hop relay communications.
Specifications for multi-hop relay communications are being prepared by a standards body. (For example, see IEEE 802.16j.) Accordingly, mobile communications services with such radio relay stations are not actually provided.
However, the above-described background technology has problems such as those described below.
An area is often dotted with points where received power is significantly reduced, depending heavily on the location of installation of a radio base station. In such points where received power is significantly reduced, the quality of radio communications is too degraded to provide services. In order to cover such dead points of radio signals where received power is significantly reduced, it is necessary to install a new radio base station or an antenna transmission and reception part in the area. Therefore, the cost of facility installation and operating cost tend to increase. In the communications industry, this increase in costs has been becoming a major problem in providing cheap data communications services.
Further, in the case of application of multi-hop relay communications, a stationary radio base station and a stationary radio relay station installed at certain locations in a service area of mobile radio communications cannot cover the service area completely. This causes points where radio propagation loss significantly increases.